Radiation shielding material and method of making same



United States Patent 3,437,602 RADIATION SHIELDING MATERIAL AND METHOD OF MAKING SAME Irving Tashlick, 54 Lorraine Terrace, Boonton, N.J., 07005, and Alan Rosensweig, 34 Treaty Road, Mount Fern, Dover, NJ. 07801 No Drawing. Filed Nov. 25, 1966, Ser. No. 596,797 Int Cl. G21f 1/08 U.S. Cl. 252478 9 Claims ABSTRACT OF THE DISCLOSURE There is disclosed herein a radiation shielding structural member, and method for making such a member, which comprises a homogeneous mixture containing a major proportion of a finely divided particulate high density metallic ore and a minor proportion of a solid organic thermoset resin that is the product of a quick cured low viscosity thermosetting liquid containing a curable polyester resin and styrene.

This invention relates to radiation shielding. More particularly, this invention relates to improved shielding material and to methods for making such material.

During recent years there has been a substantial growth in the use of equipment incorporating radioactive materials or other elements producing radiation. Such equipment is used in a variety of fields, such as industry, health services and scientific research. The equipment also is quite Varied, ranging from large scale particle accelerators to X-ray units and other devices producing radiation beams from physically small sources such as radioactive elements. The equipment must be provided with shielding, typically quite extensive, in order to protect operating personnel or sensitive equipment from the harmful effects of unwanted radiation.

Providing suitable radiation shielding at reasonable cost has presented a problem, particularly for equipment producing high-energy gamma radiation. The use of lead for shielding provides satisfactory absroption of energy, and requires but relatively small volume, but lead is too expensive to be commercially feasible for many applications. The cost of shielding can be reduced substantially by employing concrete, but concrete is much less effective than lead in absorbing radiation and therefore a much larger volume of concrete must be used to obtain the same degree of protection. In many instances, the volume of concrete required for adequate protection creates awkward problems of construction and layout of the facilities. Thus, although concrete is used extensively for such shiellding, there has existed a need for a more effective shielding material.

The basic reason that concrete is relatively ineffective in absorbing radiation is that its density is relatively low and inasmuch as water is used in the mortar, the cured concrete is porous. Generally speaking, the effectiveness of any material as an absorber of high energy gamma radiation is proportional to its density, and the density of concrete is relatively low, e.g. about 140 to 150 pounds per cubic foot, whereas lead, for comparison, has a density of over 700 pounds per cubic foot. To improve the effectiveness of concrete, attempts have been made to combine with the concrete various materials having a considerably higher density such as magnetite having a density of about 300 pounds per cubic foot.

Such so-called loaded concrete can be made with a density of perhaps 200 or more pounds per cubic foot, a significant improvement over untreated concrete with respect to absorption effectiveness. However, such loaded concrete has not been found generally satisfactory as a material for radiation shields, in part because it does not have sufficient structural strength. Also, the loading material could not be distributed uniformly throughout the concrete and as a result excessive amounts of radiation could pass through portions of the shielding containing less than average concentrations of the loading material. In curing loaded concrete, much time is consumed before it sets, thus providing opportunity for the loading material or filler to settle out, yielding a non-uniform product.

In accordance with the present invention to be described hereinbelow, an improved radiation shielding material is provided which is of relatively high and uniform' density, and which has substantial structural strength. Moreover, this material can readily be formed in a variety of shapes and sizes to meet the diverse requirements of the different kinds of equipment to be protected. For example, the material can be cast into large sections several feet in length, width or thickness, to be combined in an overlapping fashion to form a wall or door of an enclosure where radiographs are to be made of bulky items such as rocket engines; the door might, for example, be eighteen feet high and eighteen feet wide, with a thickness of several feet. Shielding material in accordance with this invention can also be used in small size units, e.g. in the form of a box having a central cavity suited for holding a source of radioactive material for transport. The shielding material to be described combines the desirable features of structural strength and plastic formability with a uniformly high density ordinarily obtainable only in more expensive materials.

In general, in accordance Wtih the invention there is provided a radiation shielding material cast or molded into the desired shape. The molded product is made by combining a high proportion of an inexpensive metallic ore in finely divided form and a low viscosity (e.g.-500 centipoises) liquid organic resin binder, present to the extent of about 2 to 10% by weight in the mixture.

As one example of the invention, a presently preferred method of making shielding material comprises, a mixture of magnetic particles, with an organic resin as a binder (consisting of polyester and styrene) in the propor tion of about 92 to parts magnetite to 8-10 parts by weight resin. Typically one starts with 7 parts of dry magnetite powder of 30 mesh. This powder is mixed with the organic resin binder until homogeneous. Thereupon, one adds to the mixture about 3 parts of magnetite powder of 120 mesh and continues mixing at a non-polymerizing temperature until the final mix is also homogeneous. A typical resin which is used is a general purpose polyester resin such as maleic-phthalic anhydride polyester (in substantially uncrosslinked form) containing about 50% by weight of styrene monomer plus a conventional amount of a polymerization catalyst (e.g. 1.25% of benzoyl peroxide). One example of such resin is that presently sold by Allied Chemical Corp., New York, New York under its trade name of Plaskon PE317; this product is indicated to be a 1 to 1 mol ratio of maleic anhydn'de and propylene glycol, containing about 50% by weight of styrene monomer. After mixing, the uncured mortar containing the ore particles and liquid resin is packed into a suitable mold, vibrated to insure filling of the mold and maximum density and then cured in an oven. Curing is preferably accomplished in two stages, such as heating for 15 minutes at 80 C. and finishing the cure for another 15 minutes at C. However, curing in one step at a polymerization temperature may also be employed.

Various types of resins may be used as binders provided they are thermosetting, quick curing (to avoid physical separation) and low in viscosity so that substantial homogeneity may be attained before, during and after curing. To be avoided are those resins which evolve water on curing (as is the case in condensation polymerization) and those which evolve large amounts of any kind of vapor on curing. Water vapor and other vapors would cause bubbles and voids in the cast structure, thus defeating the aim of achieving homogeneity and high density. Typical of such undesired resins is phenolformaldehyde. Epoxy resins are not suitable because the viscosity thereof is too high. Thus, the choice largely focuses on polyesters which can, and do, undergo addition polymerization, as for example, with styrene.

Besides the specific general purpose polyesterstyrene resin mixture described above, and presently preferred, one may also advantageously employ other commercially available general purpose low viscosity polyester resins which are currently sold in the United States. For example, one may use, in place of the maleic anhydride-propylene glycol-styrene mixture identified above as Plaskon PE 317, the following materials:

Resin mixture:

(1) Plaskon 941 (indicated to be a polyester of maleic anhydride and orthophthalic acid and glycol) Supplier Allied Chemical One may also use general purpose polyester resins made by esterifying phthalic acid or the anhydride thereof with an appropriate glycol such as ethylene glycol, propylene glycol and/or di-ethylene glycol, the ratio of acid to glycol being substantially equimolar. Additional general purpose polyesters which are useful are Aropol 7420 of Archer-Daniels-Midland Co. (based on orthophthalic acid), Vibrin 117 and Vibrin 156R of United States Rubber Co., New York, N.Y. and Laminac 4123 of American Cyanamid Co., New York, N.Y. In all the above instances, the resin mixture is to contain 40% to 60% by weight of styrene monomer, and preferably 50%.

In addition to magnetite particles or powders, other inexpensive, high density fillers may be used in the radiation shielding material of the invention. For example, barytes, ilmenite, hematite, galena, ferrophosphorus and like high density, inexpensive inorganic fillers and metallic ores may be advantageously employed, as well as combinations thereof.

It will be noted that by the practice of this invention there is provided an easily moldable material, which is inexpensive, homogeneous, quick setting and having a density (e.g. about 225 pounds per cubic foot) substantially in excess of that of conventional concrete. Materials of the invention will have a density approaching that of the filler since a relatively minor proportion of the binding resin need be used. The structural members made from the above materials also have high mechanical strength and may be made into a wide variety of forms by a simple casting technique. A further advantage of the invention, as contrasted with conventional or loaded concretes, lies in the relative speed of curing materials of the invention. Concrete is slow curing and thus productivity of a given concrete product per mold is rather W.

4 In contrast, use of the invention substantially increases productivity of a mold and thus tends to lower mold costs as a portion of the cost of a finished radiation shielding product Having now described various embodiments of the invention, what is claimed is:

1. A radiation shielding structural member which comprises a homogeneous mixture of a major proportion of a high density metallic ore in finely divided particulate form and a minor proportion of a solid organic thermoset resin, said solid resin being the quick cured product of a low viscosity thermosetting liquid which is substantially free of vapor or water evolution on curing, said thermosetting liquid containing a polyester resin diluted with a substantial proportion of styrene.

2. A structural member according to claim 1 wherein said solid resin is present to the extent of about 2% to about 10% by Weight of said member.

3. A structural member according to claim 1 wherein said ore is selected from the class consisting of magnetite,

. galena, ilmenite, hematite, baryte and ferrophosphorus.

4. A structural member according to claim 1 wherein said thermosetting liquid contains about 40% to about of said polyester resin and about 40% to about 60% of said styrene, said percentages being by weight, and a small amount of a polymerization catalyst.

5. A structural member according to claim 1 wherein said polyester resin is a substantially uncross-linked polyester of maleic acid, maleic anhydride, phthalic acid or phthalic anhydride with ethylene glycol, propylene glycol or diethylene glycol.

6. The method of making a radiation shielding structural member which comprises: (A) homogeneously mixing about to 98 percent by weight of a finely divided high density metallic ore with 2 to 10 percent of a low viscosity thermosetting liquid resin comprised of a polyester, a polymerization catalyst and a substantial proportion of cross-linking monomer, (B) casting said mixture in a mold and curing said mixture to solid form by subjecting said mixture to a polymerization temperature for a shortitme.

7. A method of making a radiation shielding structural member according to claim 6 wherein said polyester is a substantially uncross-linked polyester of maleic acid, maleic anhydride, phthalic acid or phthalic anhydride with ethylene glycol, propylene glycol or diethylene glycol.

8. A method of making a radiation shielding structural member according to claim 6 wherein said thermosetting liquid resin contains about 40% to about 60% of said polyester and about 40% to about 60% of styrene as said cross-linking monomer, said percentages being by weight.

9. A method of making a radiation shielding structural member according to claim 6 wherein said metallic ore is selected from the class consisting of magnetite, galena, ilmenite, hematite, baryte and ferrophosphorus.

References Cited UNITED STATES PATENTS 2,961,415 11/1960 Axelrad.

3,106,535 10/1963 Blanco 252478 3,142,649 7/ 1964 Blanco 252478 3,153,636 10/1964 Shanta et a1. 252478 3,173,884 3/1965 Jackson 252478 3,200,085 8/1965 Guglielmo 252478 3,230,375 1/1966 Van Wagoner et a1. 252478 X 3,247,130 4/1966 Isbell 252478 3,328,338 6/1967 Parish 252478 X CARL D. QUARFORTH, Primary Examiner.

S. I. LECHERT, Assistant Examiner. 

